1. Field of the Invention
The present invention pertains to a retrievable motor-driven bit and reamer assembly for use in drilling with well casing or drill pipe wherein the bit and reamer assembly may be retrieved without removing the casing or drill pipe from the wellbore.
2. Background
Conventional rotary drilling operations require relatively frequent withdrawal of the elongated sectionalized drillstem or "drillstring" from the wellbore to inspect or replace the bit or portions of the drillstem, to perform well logging and to install permanent well casing. This insertion and withdrawal process is time-consuming, hazardous to operating personnel and increases the possibility of damaging the well due to inadvertent dropping of the drillstring into the wellbore or encountering the influx of formation fluids into the wellbore due to the swabbing effect encountered during drillstring insertion and removal processes.
A co-pending patent application Ser. No. 07/744,852 filed Aug. 14, 1991, assigned to the assignee of the instant invention, entitled "Drilling With Casing and Retrievable Drill Bit" and filed in the name of Richard E. Leturno is directed to one improvement in retrievable drill bits with reamer or cutter arms which enlarge the wellbore to accommodate the drillstem but which may be retracted to permit retrieval of the bit assembly through the drillstem without removing the drillstem from the wellbore. The improvements described in the above-referenced patent application are particularly useful, as with the present invention, for operations in so-called drilling with casing or drill pipe which is left in the wellbore to form a casing or support structure. The present invention provides another retrievable bit assembly which utilizes a unique mechanism for providing eccentric cutting action to enlarge the wellbore to permit movement of the drill pipe or casing into the wellbore behind the bit.